The Role of the Skeletal Muscle Secretome in Mediating Endurance and Resistance Training Adaptations
←
→
Page content transcription
If your browser does not render page correctly, please read the page content below
REVIEW
published: 12 August 2021
doi: 10.3389/fphys.2021.709807
The Role of the Skeletal Muscle
Secretome in Mediating Endurance
and Resistance Training Adaptations
Aurel B. Leuchtmann, Volkan Adak, Sedat Dilbaz and Christoph Handschin*
Biozentrum, University of Basel, Basel, Switzerland
Exercise, in the form of endurance or resistance training, leads to specific molecular
and cellular adaptions not only in skeletal muscles, but also in many other organs
such as the brain, liver, fat or bone. In addition to direct effects of exercise on these
organs, the production and release of a plethora of different signaling molecules
from skeletal muscle are a centerpiece of systemic plasticity. Most studies have
so far focused on the regulation and function of such myokines in acute exercise
bouts. In contrast, the secretome of long-term training adaptation remains less
well understood, and the contribution of non-myokine factors, including metabolites,
enzymes, microRNAs or mitochondrial DNA transported in extracellular vesicles or by
other means, is underappreciated. In this review, we therefore provide an overview on
Edited by:
Domenico Di Raimondo, the current knowledge of endurance and resistance exercise-induced factors of the
Università degli Studi di Palermo, Italy skeletal muscle secretome that mediate muscular and systemic adaptations to long-
Reviewed by: term training. Targeting these factors and leveraging their functions could not only have
Vanessa Azevedo Voltarelli,
Hospital Sirio Libanes, Brazil
broad implications for athletic performance, but also for the prevention and therapy in
Tibor Hortobagyi, diseased and elderly populations.
University Medical Center Groningen,
Netherlands Keywords: skeletal muscle, exercise, myokines, PGC-1alpha, endurance training, resistance training, secretome
*Correspondence:
Christoph Handschin
christoph.handschin@unibas.ch INTRODUCTION
Specialty section: Exercise, in its various forms, provides an array of physiological stimuli that evokes metabolic
This article was submitted to and molecular perturbations in skeletal muscle as well as many other organ systems. Training
Exercise Physiology, adaptations are structural and functional changes resulting from repeated exposure to these
a section of the journal exercise-induced stimuli, leading to improved physiological capacity and decreased risk for
Frontiers in Physiology morbidity and mortality (Egan and Zierath, 2013). The detailed molecular processes induced by
Received: 14 May 2021 different exercise stimuli are, however, still poorly characterized. In particular, it is incompletely
Accepted: 23 July 2021 understood how the signals induced by acute exercise bouts translate into specific adaptations
Published: 12 August 2021
following long-term training (Sanford et al., 2020). A sound understanding of the mechanisms
Citation: underlying the adaptation to exercise training and the identification of molecular effectors of
Leuchtmann AB, Adak V, Dilbaz S the adaptive response is essential—not only for individuals with athletic ambitions, but also to
and Handschin C (2021) The Role
infer potential clinical implications and to develop novel therapeutic avenues for geriatric and
of the Skeletal Muscle Secretome
in Mediating Endurance
diseased populations.
and Resistance Training Adaptations. Endurance and resistance exercise are the two extremes of a broad continuum of exercise
Front. Physiol. 12:709807. modalities and elicit distinct but also overlapping training responses (Egan and Zierath, 2013).
doi: 10.3389/fphys.2021.709807 Acutely, both endurance and resistance exercise trigger the systemic release of a plethora of
Frontiers in Physiology | www.frontiersin.org 1 August 2021 | Volume 12 | Article 709807Leuchtmann et al. Myokines in Exercise Training Adaptation
bioactive molecules such as proteins, metabolites and microRNAs number of myokines have been described in various systems and
(Whitham et al., 2018; Morville et al., 2020; Vechetti et al., 2021). paradigms, but only a subset of these is regulated by exercise
The major physiological function of this release is thought to be (also called “exerkines”) (Laurens et al., 2020). In the following
the maintenance of metabolic homeostasis i.e., the coordination sections, the exerkines that are potentially involved in long-term
of nutrient sensing, delivery, uptake and utilization, which endurance training adaptation are described.
requires an extensive crosstalk between almost every organ within
the body (Murphy et al., 2020). Since the discovery of myokines, Interleukin-6
skeletal muscle has been appreciated as an endocrine organ, The discovery that skeletal muscle contributes to the exercise-
whose secreted factors are strongly involved in coordinating induced increase in circulating interleukin-6 (IL-6) (Steensberg
the metabolic network during acute exercise bouts in order et al., 2000) initiated the concept of myokines and muscle as
to optimize energy substrate availability in working muscles an endocrine organ. Despite initial skepticism about the muscle
and to sustain force production. Accordingly, during the last cell origin, there is now good evidence that IL-6 is transcribed,
few decades, large efforts have been taken to elucidate the translated and released from myofibers during exercise (Hiscock
components of the muscle fiber secretome, with a particular et al., 2004; Whitham et al., 2012). The mechanism of secretion in
focus on the regulation of the metabolic homeostasis during contrast has not been entirely elucidated so far, although different
and after acute exercise bouts. In contrast, unraveling which candidate pathways have been proposed (Hojman et al., 2019).
of the acutely secreted factors are involved in mediating local IL-6 serves primarily as a metabolic coordinator during acute
and systemic adaptations to long-term training has received exercise bouts by regulating lipolysis in adipose tissue (Van Hall
much less attention. In this review, we therefore summarize et al., 2003) and hepatic glucose production (Febbraio et al., 2004)
the current knowledge about secreted factors stimulated by coordinated with substrate uptake and utilization in skeletal
endurance and/or resistance exercise that are contributing to muscle (Chowdhury et al., 2020). The secretion of IL-6 is tightly
long-term training adaptations (Figure 1). linked to the metabolic state, e.g., pre-exercise muscle glycogen
content or oral glucose availability (Starkie et al., 2001; Steensberg
et al., 2001).
ENDURANCE TRAINING ADAPTATION IL-6-driven lipolysis during acute exercise bouts may lead to
improved body composition following endurance training over
Endurance exercise is the training modality of choice when time. In fact, the reduction of visceral and epicardial fat tissue
aiming for improvements in cardiorespiratory, cardiovascular in response to 12 weeks of high-intensity endurance training
and metabolic function (Mcgee and Hargreaves, 2020). was dependent on intact IL-6 signaling, at least in a cohort of
Endurance exercise-induced stresses elicit multiple signaling obese and previously sedentary individuals (Christensen et al.,
events in skeletal muscle, heart and the respiratory system, 2019; Wedell-Neergaard et al., 2019). IL-6 is further known for
which exert their effects on training adaptation in a cell-intrinsic its anti-inflammatory effects in both acute exercise bouts and
manner. In addition, components of the skeletal muscle exercise training. The acute effects are primarily due to the IL-
secretome, the entity of proteins, peptides, metabolites and other 6-triggered production of the cytokines IL-1 receptor antagonist
molecules (Weigert et al., 2014; Florin et al., 2020), released upon and IL-10, which, amongst other exercise-induced factors, create
acute endurance exercise bouts, may also act as mediators of an anti-inflammatory systemic environment (Pedersen, 2017).
training adaptation through autocrine, paracrine and endocrine The long-term effects of training, in addition, may be positively
mechanisms. Many of the molecular adaptations in endurance influenced by the above-described reduction in visceral adipose
exercise are controlled by the peroxisome proliferator-activated tissue, a main contributor to the chronic systemic low-grade
receptor γ coactivator 1α (PGC-1α). This coregulator protein inflammatory state observed in the context of obesity (Severinsen
is activated by exercise-linked signaling in muscle and in turn and Pedersen, 2020). IL-6 may further be involved in endurance
coordinates and orchestrates the transcription of a complex training-induced cardiac remodeling, as intact IL-6 signaling
network encoding several biological programs linked to was required for training-linked left ventricle hypertrophy
endurance training, including mitochondrial function, oxidative (Christensen et al., 2019). However, changes in stroke volume
metabolism, calcium homeostasis and contractile properties or end-diastolic volume (left ventricular ejection fraction) were
of slow muscle fibers, or angiogenesis (Kupr and Handschin, not affected, and VO2peak , a marker for peak endurance capacity,
2015). Intriguingly, many myokines are also under the control of was similarly ameliorated after endurance training in participants
PGC-1α (Schnyder and Handschin, 2015). In the first part of this treated with the IL-6 receptor (IL-6R) antagonist tocilizumab
review, we will discuss how signaling factors are regulated and or placebo (Wedell-Neergaard et al., 2019; Ellingsgaard et al.,
involved in the control of endurance training adaptation. 2020). As a caveat: in the IL-6R blocking studies, it is not clear
whether the long-term adaptations are in fact due to muscle-
secreted IL-6 as other organs could contribute to the systemic
MYOKINES increase during exercise. Furthermore, there is no evidence of
sustained or enhanced IL-6 myokine production and release
By definition, myokines are peptides that are produced and in endurance-trained muscle, implying that if long-term effects
secreted by skeletal muscle fibers and subsequently exert auto-, are mediated by muscle IL-6, they would be the consequences
para- or endocrine effects (Severinsen and Pedersen, 2020). A vast of repeated acute elevation and related signaling. In fact, the
Frontiers in Physiology | www.frontiersin.org 2 August 2021 | Volume 12 | Article 709807Leuchtmann et al. Myokines in Exercise Training Adaptation
acute exercise-induced rise in systemic IL-6 levels and muscular following training and thereby facilitated energy substrate and
IL-6 mRNA are diminished by training, although potentially oxygen supply as well as byproduct removal. Besides ample
counteracted by an enhanced muscular expression of the IL-6R post-transcriptional regulation of VEGF stability, in many cell
in trained muscle (Severinsen and Pedersen, 2020). types VEGF induction is strongly regulated by the canonical
hypoxia response pathway and hypoxia-inducible factor 1 alpha
Interleukin-8 (HIF-1α)-controlled transcription (Arcondeguy et al., 2013).
IL-8 is primarily known for its angiogenic activity and role as a However, exercise-induced VEGF expression in skeletal muscle
chemoattractant in inflammatory processes (Harada et al., 1994; can be driven by additional, HIF-1α-independent mechanisms,
Keane et al., 1997). IL-8 acting as a potential myokine has been orchestrated by PGC-1α interacting with the transcription factors
under investigation since in vitro studies have demonstrated that estrogen-related receptor α (ERRα) and activator protein-1 (AP-
it is produced and secreted by muscle cells (De Rossi et al., 1) (Arany et al., 2008; Baresic et al., 2014). In addition, PGC-
2000). Similar to IL-6, lower muscle glycogen content favors 1α-controlled angiogenesis may involve enhanced expression
IL-8 transcription in human skeletal muscle, while changes in of platelet-derived growth factor (PDGF)-BB and angiopoietin
plasma levels due to exercise, conversely, seem to be glycogen- 2 as well as secreted phosphoprotein 1 (SPP1). PDGF-BB
independent (Nieman et al., 2003; Chan et al., 2004). Moreover, a recruits mural cells to support and encase the endothelium
substantial increase in plasma IL-8 appears to require prolonged, and angiopoietin 2 facilitates sprouting, while SPP1 instructs
potentially damaging muscle actions such as marathon- or macrophages to signal to adjacent ECs, pericytes and smooth
ultramarathon-type of running (Nieman et al., 2002, 2003; Suzuki muscle cells thereby orchestrating angiogenesis in concert with
et al., 2003) and is less frequently observed or lower in magnitude VEGF (Arany et al., 2008; Rowe et al., 2014).
following short term (Mucci et al., 2000) or less damaging
muscular activity such as cycling or rowing (Henson et al., 2000; Musclin
Chan et al., 2004). Considering the systemic stress and muscle Skeletal muscle-derived musclin was first identified by screening
damage induced by a marathon, the chemoattractant properties cDNA libraries of mouse skeletal muscle for putative secreted
of IL-8 in inflammatory responses and immune cell activation proteins (Nishizawa et al., 2004). More than a decade later,
are likely predominant in this context. Although muscle fiber- Subbotina et al. (2015) established musclin as an exercise-
secreted IL-8 cannot be fully precluded to contribute to the controlled myokine whose production and systemic release is
chemoattraction of neutrophils and macrophages recruited for driven by Ca2+ -dependent activation of Akt and subsequent
muscle repair, systemic IL-8 during strenuous exercise might nuclear exclusion of forkhead box O1 transcription factor
also be derived from other cell types resident in skeletal (FoxO1), a known transcriptional inhibitor of the musclin-
muscle or beyond. encoding gene osteocrin (Ostn) in skeletal muscle (Yasui et al.,
IL-8 signals via the CXC receptor 2 (CXCR2), which is 2007). In sedentary mice, ubiquitous disruption of Ostn leads
expressed by endothelial cells (ECs) and affects EC proliferation to impaired treadmill performance, reduced voluntary wheel
and capillary tube organization (Li et al., 2003). Therefore, running and lower succinate dehydrogenase activity in skeletal
a potential role in training-induced capillarization has been muscle, but had no effect on oxygen consumption during
proposed, which is supported by the observation that CXCR2 exercise or mitochondrial and respiratory complex protein
mRNA and protein levels are upregulated in vascular ECs upon content in skeletal muscle (Subbotina et al., 2015). In contrast,
exercise (Frydelund-Larsen et al., 2007), but conflicting with the when exposed to short-term (5 days) treadmill training, all
impaired capillary outgrowth in human muscle explants exposed variables described above were significantly lower in Ostn-
to higher levels of IL-8 (Amir Levy et al., 2015). The potentially KO compared to wild-type (WT) animals, indicating impaired
pro-angiogenic effects of IL-8 on capillarization are likely to training adaptation in musclin deficient mice. Unfortunately,
be dose-dependent and perhaps require the pulsatile nature of however, longer training interventions and pre- vs. post-training
exercise training. comparisons of running capacity and oxygen consumption were
not performed in this study, which makes it difficult to draw firm
Vascular Endothelial Growth Factor conclusions about the importance of muscle derived musclin in
Vascular endothelial growth factor (VEGF) is among the most endurance training adaptation.
important pro-angiogenic factors in many tissues. Accordingly, The study by Subbotina et al. (2015) furthermore suggested
VEGF transcription and translation are highly induced in that musclin acts in concert with atrial natriuretic peptide
skeletal muscle upon endurance exercise (Jensen et al., 2004a; (ANP) to co-stimulate cGMP production in skeletal muscle,
Olfert et al., 2016). Systemic levels of VEGF, on the other which has been linked to PGC-1α-dependent mitochondrial
hand, often do not increase strongly during exercise, mainly biogenesis (Nisoli et al., 2004). Exercise-induced increase in
because VEGF accumulates in the muscle interstitium acting both cGMP and PGC-1α mRNA was blunted in muscles of
on vascular ECs to trigger blood vessel formation (Höffner Ostn-KOs (Subbotina et al., 2015). As endocrine musclin release
et al., 2003; Jensen et al., 2004a; Landers-Ramos et al., 2014). increases systemic levels of various NPs by competing for their
Because muscle cells can produce and secrete VEGF in response clearance receptor (Kita et al., 2009), exercise-induced musclin
to electrostimulation (Jensen et al., 2004b), muscle fibers are potentially modulates the action of these peptides. NPs have a
thought to autonomously react to the stresses induced by exercise wide spectrum of target tissues including skeletal muscle, heart,
by secreting VEGF, which leads to enhanced capillarization bone and kidneys. Thus, the endocrine release of musclin may,
Frontiers in Physiology | www.frontiersin.org 3 August 2021 | Volume 12 | Article 709807Leuchtmann et al. Myokines in Exercise Training Adaptation
for example, increase cardiac NPs involved in heart remodeling, endurance training, muscular apelin expression and increased
which in addition to the described effects on skeletal muscle could plasma apelin levels has been proposed (Besse-Patin et al., 2014;
explain the reduced V̇O2peak observed in endurance-trained Ostn Fujie et al., 2014). In a comprehensive series of experiments,
KO compared to WT animals. Vinel et al. (2018) investigated the muscular contribution
to exercise-induced plasma apelin as well as the resulting
B-Type or Brain Natriuretic Peptide local and systemic effects. In vitro, apelin was found to be
Brain natriuretic peptide (BNP) is mainly produced by secreted by contracting human myotubes, and in mice, hindlimb
cardiomyocytes in cardiac ventricles upon stretch to initiate venous-arterial difference of apelin increased following muscle
signaling events that reduce blood pressure and blood volume contractions induced by sciatic nerve stimulation. Moreover,
(Goetze et al., 2020). As it is able to reduce fibrosis in the liver skeletal muscle-specific knockdown of apelin expression through
(Sonoyama et al., 2009) and potentially in the heart (Wang et al., viral vectors did not affect resting levels, but impaired exercise-
2004), coupled with the described immunomodulatory effect induced elevation of plasma apelin, reaffirming the strong
in macrophages (Chiurchiu et al., 2008), BNP is involved in contribution of skeletal muscle to plasma apelin during exercise.
inflammatory responses related to tissue repair and regeneration. The muscle apelin knockdown also led to atrophy and reduced
In humans, plasma BNP increases in response to exercise (Ohba grip strength, grid-hanging time and treadmill performance,
et al., 2001; Huang et al., 2002), but the relative contribution all of which were rescued with daily apelin treatment. While
of different tissues to this increase is not exactly known. apelin supplementation had synergistic effects on endurance
Because BNP expression is much higher in cardiac compared to training and further improved fatigue resistance, muscle-specific
skeletal muscle, the majority of circulating BNP likely originates overexpression phenocopied the functional effects of exercise
from the heart. Nevertheless, by combining in vitro secretome training concomitant with an increased mass of overexpressing
analyses with in vivo experiments, BNP has been identified muscles. Regarding potential underlying mechanisms, Vinel
as a PGC-1α1/ERRα-driven myokine induced upon exhaustive et al. (2018) proposed an autocrine regulatory loop, in which
endurance exercise in mouse skeletal muscle (Furrer et al., local apelin production enhances muscular endurance by
2017). Together with other exercise and/or PGC-1α-regulated increasing intramyofibrillar mitochondrial number and function
factors with paracrine immunomodulatory function in skeletal in part via AMP-dependent protein kinase (AMPK) activation.
muscle, BNP may mediate the active engagement of skeletal A previous study in insulin-resistant high-fat diet mice had
muscle resident immune cells e.g., by orchestrating M1 to M2 already suggested a role for the apelin-AMPK axis in skeletal
transition in macrophages (Furrer et al., 2017). Whether and muscle mitochondrial biogenesis and oxidative capacity, as both
how the BNP-driven transient activation of macrophages after were augmented by apelin treatment in an AMPK-dependent
acute endurance bouts aids training adaptation in the long-term manner (Attané et al., 2012).
remains to be investigated. It seems, however, plausible that BNP- Apelin/apelin receptor signaling plays a key role in the
activated macrophages facilitate muscle repair and regeneration development and organization of vascular networks, as evidenced
following acute exercise bouts and/or confer enhanced capacity in various in vitro and in vivo models (Masri et al., 2004;
to trained muscle to respond to exercise-induced insults by Kasai et al., 2008; Kidoya et al., 2008, 2015; Kunduzova
increasing their resident numbers. While the contribution et al., 2008). In addition, compared to WT animals, apelin-
of cardiac BNP most likely supersedes that of muscle BNP transgenic mice exhibit increased EC numbers in skeletal muscle
systemically, the latter could also have autocrine functions in when fed a high-fat diet, while animals on a standard diet
skeletal muscle, based on the widely described actions of NPs displayed higher oxygen consumption and upregulated mRNA
on skeletal muscle (Miyashita et al., 2009; Engeli et al., 2012; expression of endothelium-specific receptor tyrosine kinase 1 and
Subbotina et al., 2015). 2, which are important for vascular maturation (Yamamoto et al.,
2011). Mechanistically, apelin signaling pushes ECs into a pro-
Apelin angiogenic state by enhancing intracellular glycolytic activity,
Apelin (short for APJ Endogenous Ligand) was first described a process mainly driven by PFKFB3 and c-MYC, two central
as a peptide purified from bovine stomach extracts that binds regulators of EC metabolism (De Bock et al., 2013; Wilhelm
to the orphan APJ G protein-coupled receptor (GPCR), now et al., 2016; Helker et al., 2020). The apelin protein required for
known as apelin receptor (Tatemoto et al., 1998). Both the the pro-angiogenic switch, however, may be primarily derived
apelin peptide and its receptor are expressed in many tissues from tip ECs for autocrine stimulation, as hypoxia can directly
including different brain regions and insulin-responsive tissues trigger apelin transcription in ECs by activating HIF-1α, which
such as adipose tissue, skeletal muscle, heart and liver (Castan- subsequently binds to hypoxia-responsive elements present in
Laurell et al., 2012). Apelin has long been known for its the apelin gene (Cox et al., 2006; Eyries et al., 2008; Del Toro
adipokine function, induced by insulin and modulating glucose et al., 2010). Similarly, during skeletal muscle regeneration,
and lipid metabolism as well as insulin sensitivity, which is apelin is secreted together with oncostatin M and periostin
why pharmacological targeting of the apelin receptor gained by ECs and myogenic progenitor cells to couple myo- and
much interest for the treatment of type two diabetes (T2D) angiogenesis (Latroche et al., 2017). However, no study has
and metabolic diseases (Castan-Laurell et al., 2012). However, so far investigated whether muscle fiber secreted apelin could
skeletal muscle-derived apelin and its responsiveness to exercise mechanistically contribute to the formation of capillaries in
has only recently started to be explored, after a link between exercise-trained muscle.
Frontiers in Physiology | www.frontiersin.org 4 August 2021 | Volume 12 | Article 709807Leuchtmann et al. Myokines in Exercise Training Adaptation
Myonectin of neuronal cells. Accordingly, BDNF and its receptors are
During the characterization of the metabolic function of highly expressed in various brain regions (Huang and Reichardt,
complement component 1q/TNF-related protein (CTRP) family 2001). BDNF has been consistently reported to increase in the
members, CTRP15 (now referred to as myonectin) was identified circulation in response to exercise [e.g., (Ferris et al., 2007;
as a potential myokine (Seldin et al., 2012). In mice given access to Saucedo Marquez et al., 2015)], with the brain or blood platelet
running wheels for 2 weeks or subjected to 4 weeks of treadmill precursors (megakaryocytes) likely to constitute the largest
training, circulating myonectin levels as well as skeletal muscle contributors to this increase (Rasmussen et al., 2009; Chacón-
gene and protein expression increased substantially (Seldin et al., Fernández et al., 2016). However, both BDNF transcription
2012; Otaka et al., 2018). Furthermore, transcriptional activation and translation are induced in skeletal muscles during exercise,
of myonectin is induced by raising intracellular cAMP or Ca2+ and the BDNF protein appears to be secreted, although only
levels through either muscular activity or pharmacologic agents in relatively small amounts (Pedersen et al., 2009; Ogborn and
(Seldin et al., 2012). Since myonectin is poorly expressed in Gardiner, 2010). Therefore, muscle-derived BDNF is thought
tissues other than skeletal muscle, the main source of the training- to be mostly involved in autocrine and paracrine signaling e.g.,
induced increase in circulating levels is considered skeletal to promote muscle fiber fat oxidation via AMPK activation
muscle (Seldin et al., 2012; Otaka et al., 2018). In a combination (Matthews et al., 2009) and to regulate muscle development
of in vitro experiments and studies in mice treated with and regeneration, at least when SC-derived (Mousavi and
recombinant protein, systemic myonectin was shown to act on Jasmin, 2006; Clow and Jasmin, 2010; Miura et al., 2012). In
the liver to increase the uptake of free fatty acids by upregulating addition, a potential trophic action of muscle-derived BDNF
local lipid binding and transport proteins (Seldin et al., 2012) on innervating motor neurons (MNs) via retrograde signaling
and to suppress autophagy by activating the mammalian target of has been proposed (Koliatsos et al., 1993). Surprisingly, even
rapamycin complex 1 (mTORC1) pathway (Seldin et al., 2013). though BDNF induction seems to be a physiological response
Exercise mitigates ischaemia-reperfusion (IR) injury, to endurance exercise, muscle-specific deletion of the Bdnf
which occurs after acute ischemic events such as myocardial gene increases running performance, at least in part due to a
infarction or suppression of blood flow during cardiac surgery neuromuscular junction (NMJ) remodeling associated with a
(Moreira et al., 2020). Besides intracellular stress-defense myosin heavy chain (MyHC)-IIB to MyHC-IIX muscle fiber
mechanisms triggered directly in cardiomyocytes, the protective transition (Delezie et al., 2019). However, the role of BDNF
effect of exercise on IR injury may additionally be mediated induction (or repression) in training-induced remodeling of
by the exercise-induced myonectin release from skeletal muscle fibers or their innervating MNs through auto- and
muscle. Mice deficient for the myonectin gene displayed paracrine actions, respectively, remains to be explored.
larger myocardial infarct sizes, cardiac dysfunction and more Many chronic diseases including T2D and cardiovascular
apoptotic cardiomyocytes compared to WT animals, while disease, but also neurological disorders such as impaired
transgenic overexpression further reduced myocardial damage cognition, dementia and depression are associated with lower
after IR (Otaka et al., 2018). Intriguingly, endurance training circulating BDNF levels, and all these diseases can be mitigated
decreased infarct size after IR in WT, but not in myonectin with exercise interventions (Pedersen et al., 2009). Moreover, in
KO mice, supporting the notion that the training-induced mouse models, exercise-induced cathepsin B release from skeletal
elevation of circulating myonectin confers this protective effect muscle elevate the abundance of BDNF in the hippocampus,
(Otaka et al., 2018). which was linked to neuroprotection and improved memory
Regarding human myonectin, there are so far only reports function (Moon et al., 2016). The cathepsin B-mediated muscle-
about plasma levels, and evidence as to the effect of both acute brain crosstalk is further supported by human data, as training
and chronic exercise is conflicting. In heterogeneous populations, induced cathepsin B transcription and systemic protein levels
some studies observed an increase while others reported either correlated with hippocampus-dependent memory function in
no change or even a decrease in circulating myonectin (Lim et al., healthy young adults (Moon et al., 2016). For more detailed
2012; Pourranjbar et al., 2018; Kamiński et al., 2019; Bahremand information on muscle-brain crosstalk in the context of exercise,
et al., 2020). Thus, more research is required, including studies on the interested reader is referred to a recent review on the topic
muscle biopsies, to help elucidating the role of human myonectin (Delezie and Handschin, 2018).
in general. Moreover, even though loss of myonectin has no Neurturin, a member of the glial cell line-derived
effect on running performance in untrained mice (Little et al., neurotrophic factor (GDNF) family, is a PGC-1α1-regulated
2019), there are no data available on post-training performance myokine and involved in neuromuscular synapse maturation
to conclude about potential impairments in training adaptation. during development and regeneration (Baudet et al., 2008).
Accordingly, neurturin promotes neuromuscular junction (NMJ)
formation in an in vitro microfluidics cell co-culture system
Brain-Derived Neurotrophic Factor and (Mills et al., 2018b). Moreover, recent evidence indicates that
Neurturin neurturin could be one of the PGC-1α1 effector myokines that
Brain-derived neurotrophic factor (BDNF) signals via couples increased muscle oxidative capacity to corresponding
tropomyosin receptor kinase B (TrkB) and p75NRT receptors changes in MN properties (Correia et al., 2021). While increased
and is a member of the neurotrophin (NT) family, a class of muscular expression after endurance exercise was detected in
secretory factors that regulate survival, growth and function mice and humans (Schlittler et al., 2019; Correia et al., 2021),
Frontiers in Physiology | www.frontiersin.org 5 August 2021 | Volume 12 | Article 709807Leuchtmann et al. Myokines in Exercise Training Adaptation ectopic expression of neurturin in mouse skeletal muscle was of total BAIBA or the fact that BAIBA has a chiral center sufficient to induce a shift toward slower MN and NMJ features, and therefore two different enantiomers, which may lead to while at the same time, the muscles of these mice displayed a interindividual differences in their relative distribution and hence phenotype similar to PGC-1α1 overexpressing animals. Like the relative change upon exercise (Stautemas et al., 2019). BDNF, muscle-derived neurturin might primarily act in an auto- Human meteorin-like plasma levels were significantly and paracrine manner, as it was undetectable in the plasma of increased after exercise in temperate (24–25◦ C) and warm overexpressing mice (Correia et al., 2021). (36.5–37.5◦ C) water, but decreased in cold (16.5–17.5◦ C) water On the muscle side, neuromuscular adaptations to exercise in a cohort of overweight women (Saghebjoo et al., 2018). training likely reflect the integration of the local relative In mice, a recent study reported that meteorin-like facilitates abundance of various neurotrophic factors and their receptors. skeletal muscle repair by activating signal transducer and For example, GDNF is induced by exercise in rat skeletal muscle activator of transcription 3 (STAT3) signaling in macrophages and has been implicated in enhanced NMJ endplate area in to induce insulin-like growth factor 1 (IGF-1) production, various training paradigms (Nguyen et al., 1998; Gyorkos and which in turn had a direct effect on muscle SC proliferation Spitsbergen, 2014; Gyorkos et al., 2014). Besides BDNF, other (Baht et al., 2020). However, in this context, meteorin-like was neurotrophic factors (NTs) such as nerve growth factor (Capsoni released from infiltrating macrophages rather than resident cells et al., 2000), NT-3 (Ernfors et al., 1994; Klein et al., 1994) or muscle fibers. or N-4/5 (Belluardo et al., 2001) are found in skeletal muscle and exert important functions in neuromuscular physiology. However, their regulation by exercise as well as their potential MITOCHONDRIAL-DERIVED PEPTIDES functions in the training response in rodents or humans remain largely elusive (Lippi et al., 2020). Collectively, however, the Mitochondria possess a distinct circular genome that gives rise to regulation, secretion and function of such factors imply a 13 oxidative phosphorylation complex subunits. However, short surprisingly broad retrograde signaling from skeletal muscle that open reading frames (sORF) have recently been discovered to leads to a remodeling of the NMJ and potentially MN phenotype, encode additional biologically active small peptides (
Leuchtmann et al. Myokines in Exercise Training Adaptation
an extracellular receptor, through direct interaction or by other performance, muscle function and oxidative capacity in SUCNR1
means could not be determined. While putative cell-surface KOs is required to fully determine the role of succinate signaling
receptors for humanin have been suggested (Lee et al., 2013), in endurance training adaptation.
extracellular receptors for MOTS-c or other MDPs are still
elusive. Although more research has to be conducted, thus far
it seems more likely that intracellular interactions dominate the MicroRNAs AND EXTRACELLULAR
effects of MDPs on metabolic homeostasis in the context of VESICLES
exercise training (Kim et al., 2018).
MicroRNAs (miRNAs) are small non-coding single-stranded
RNA molecules that modulate gene expression at the post-
SUCCINATE transcriptional level. Extracellular miRNAs are either bound
to protein complexes associated with high-density lipoproteins
TCA cycle intermediates such as fumarate, malate, citrate and or located inside small extracellular vesicles (EVs). EVs are
succinate have long been known to accumulate in skeletal muscle generated by most cell types and facilitate the exchange of
during exercise (Sahlin et al., 1990; Gibala et al., 1997). Of these, miRNAs and other biological components among cells and
succinate is further released into the blood stream in substantial tissues (Groot and Lee, 2020). Endurance exercise triggers a
amounts (Hochachka and Dressendorfer, 1976). On target cells, systemic EV release, and the amount as well as the specific
succinate acts as an extracellular signaling molecule by binding to composition of the miRNA cargo appears to be dependent on the
the GPCR succinate receptor 1 (SUCNR1) (He et al., 2004; Regard exercise modality and intensity (Vechetti et al., 2021). Vechetti
et al., 2008) and, in muscle fibers, stimulates monocarboxylate et al. (2021) further analyzed previously published miRNAs
transporter (MCT)1-facilitated, pH-gated release of succinate isolated from plasma EVs after a single bout of exercise and ran a
(Reddy et al., 2020). A wide array of physiological and prediction analysis in order to both determine possible targets of
pathological processes have now been described to be, at least miRNA carrying EVs and to infer their potential systemic effects.
in part, regulated by extracellular succinate and/or SUCNR1 The top two most significantly enriched biological processes
signaling including energy expenditure, inflammation, blood revealed by gene ontology analysis were the response to reactive
pressure and ischemia (Tretter et al., 2016; Mills et al., 2018a). oxygen species and insulin secretion. This is in line with a recent
To evaluate the role of succinate signaling in training- study in mice showing that miRNA-containing EVs isolated
induced muscle remodeling, Reddy et al. (2020) subjected from high-intensity interval-trained muscles improved glucose
SUNCR1 KO mice to a progressive resistance wheel-running tolerance when administered to sedentary mice (Castaño et al.,
regimen. After 3 weeks, SUNCR1 KOs had accumulated larger 2020). However, due to the inability to label and track EVs
running distances compared to WT animals, indicating a released from specific cells or tissues, the relative contribution
superior endurance training response with impaired succinate of skeletal muscle to circulating EVs induced by exercise is still
signaling. On the other hand, while there was no difference at mostly speculative (Nederveen et al., 2020).
baseline, training-induced gains in grip strength were completely Besides the endocrine action of skeletal muscle-derived
compromised in SUNCR1 KOs. In line with these observations, miRNAs as circulating EV cargo, in vitro evidence suggests that
training resulted in increased neural-specific tubulin staining myofibers use miRNA secretion to regulate local processes such as
only in WT mice, while SUNCR1 KOs displayed downregulated myogenic differentiation and angiogenesis (Forterre et al., 2014;
fast MyHC isoforms as well as proteins involved in ECM De Gasperi et al., 2017; Nie et al., 2019; Mytidou et al., 2021).
organization relevant for force transmission. However, these Although it appears conceivable that a miRNA-based paracrine
results are in conflict with an earlier study, which found succinate crosstalk could be involved in mediating muscular adaptations
supplementation to increase endurance exercise capacity and to endurance training (e.g., muscle fiber capillarization or ECM
to induce an oxidative (i.e., slow MyHC dominant) fiber type remodeling), the skeletal muscle miRNA field has only recently
shift in a SUCNR1-dependent manner (Wang et al., 2019). Of started to develop and hence there are currently very few
note, while the effects of dietary succinate were attributed to studies investigating myofiber miRNA release in the context of
myocellular SUCNR1 signaling, Reddy et al. (2020) were unable endurance training adaptation.
to detect SUCNR1 in cultured muscle cells or muscle fibers
in tissue cross-sections, but found it to be highly expressed
in resident stromal, endothelial and satellite cell populations, FACTORS SECRETED BY RESIDENT OR
suggesting that succinate rather acts in a paracrine manner. The INFILTRATING CELLS IN SKELETAL
SUCNR1-dependent transcriptome in isolated cell populations MUSCLE
immediately after an acute treadmill bout revealed decreased
expression of transcripts involved in axon guidance, neuronal The complexity of skeletal muscle is often underappreciated and
projections and muscle regeneration in SUCNR1 KO mice. thus past efforts to identify factors mediating exercise-training
Thus, despite the seemingly favored endurance adaptation, adaptation have mainly focused on the secretome of myofibers.
paracrine succinate signaling appears to be required for ECM However, besides syncytial myofibers, skeletal muscle harbors
remodeling and adaptions in muscle innervation in response to a variety of mononucleated cell populations such as satellite
training. However, a thorough examination of changes in exercise cells (SCs), fibro-adipogenic progenitors (FAPs), macrophages,
Frontiers in Physiology | www.frontiersin.org 7 August 2021 | Volume 12 | Article 709807Leuchtmann et al. Myokines in Exercise Training Adaptation
neutrophils, ECs, B-, T- and glial cells (Giordani et al., 2019). only name a few—are also elicited by resistance training, in both
Exercise can induce changes in the proportion of these cell young and older individuals (Hughes et al., 2018). Although the
types as well as affect activation, polarization and secretory resistance exercise stimuli, their molecular sensors as well as the
profiles (Rubenstein et al., 2020). Muscle-resident mononuclear downstream signaling events that lead to the above described
cells contribute to the secretome of skeletal muscle and the phenotypic alterations induced by resistance training are still
adaptation to exercise training may depend on coordinated poorly characterized (Wackerhage et al., 2019), factors secreted
communication between the different cell types and muscle by skeletal muscle fibers could play a key role in modulating
fibers, similar to developmental and regenerative processes. For neuromuscular plasticity in response to resistance training.
example, IL-13 is secreted by type 2 innate lymphoid cells in Therefore, in the following sections, we provide an overview
endurance exercise, and induces signaling in myofibers that on resistance exercise-induced factors of the skeletal muscle
promotes an oxidative, high-endurance phenotype (Knudsen secretome with potential functions in mediating muscular and
et al., 2020). Accordingly, besides reduced treadmill running systemic adaptations to resistance training through autocrine,
capacity at baseline, the training-induced oxidative fiber type paracrine or endocrine actions.
shift, improvements in mitochondrial respiration, endurance
capacity and glucose tolerance were abolished in mice deficient IGF-1
for the Il13 gene. Moreover, intramuscular delivery of IL- IGF-1 is an extensively studied regulator of muscle growth,
13 recapitulated the metabolic reprogramming induced by differentiation and regeneration. Upon binding to its receptor,
endurance training and increased exercise performance. Second, IGF-1 induces phosphatidylinositol 3-kinase (PI3K)/Akt-
in recent years, histamine has emerged as a potentially important dependent pathways, which subsequently lead to the activation
mediator of the exercise response, in both acute and chronic of mTORC1 and p70S6 kinase, key regulators of protein synthesis
settings. In humans, histamine is released locally within exercised and muscle growth, at least in certain contexts and paradigms
muscles and although not entirely clear, degranulating mast cells (Goodman, 2019). Endocrine-acting IGF-1 is primarily secreted
are a likely source of post-exercise histamine (Halliwill et al., 2013; by the liver upon growth hormone (GH) stimulation, while
Romero et al., 2017). Histamine-driven alterations in skeletal skeletal muscle expresses the two mainly auto- and paracrine-
muscle gene expression could make up > 25% of the acute acting isoforms IGF-1Ea and mechano-growth factor (MGF, or
endurance exercise responsive transcriptome (Romero et al., IGF-1Eb in rodents and IGF-1Ec in humans), of which MGF is
2016). Pharmacological inhibition of the histamine H1 and H2 most responsive to mechanical signals (Goldspink, 2005; Barton,
receptors during 6 weeks of cycling interval training in healthy 2006). PGC-1α4, a shorter PGC-1α isoform, which induces
males resulted in impaired improvements of exercise capacity, a hypertrophic phenotype when overexpressed in skeletal
glycemic control and vascular function (Van Der Stede et al., muscle of mice, has been proposed to act in part through IGF-1
2021). Finally, macrophage-derived growth differentiation factor induction and myostatin suppression (Ruas et al., 2012). Systemic
3 (GDF3) (Varga et al., 2016) and EC-secreted lactate (Zhang IGF-1 administration is sufficient to induce muscle hypertrophy
et al., 2020) help in muscle regeneration upon damage, thereby and resistance training-induced gains can be further potentiated
potentially contributing to training adaptation. when combined with IGF-1 treatment (Lee et al., 2004).
However, a functional IGF-1 receptor appears to be dispensable
for muscle hypertrophy to occur in a muscle-overload context
RESISTANCE TRAINING AND THE (Spangenburg et al., 2008). Moreover, although acute resistance
SKELETAL MUSCLE SECRETOME exercise increases systemic levels of IGF-1 and other anabolic
hormones in human participants, acute raises in circulating
Increasing or maintaining skeletal muscle mass and strength is IGF-1 after resistance exercise bouts neither enhance muscle
not only essential for athletic performance, but also associated protein synthesis nor correlate with skeletal muscle hypertrophy
with increased quality of life, while conversely, a reduction in and strength gains following training (West et al., 2009; Morton
muscle mass and strength elevates the risk of (multi-)morbidity et al., 2016). The relative contribution of skeletal muscle-derived
and all-cause mortality (Furrer and Handschin, 2019). Resistance IGF-1 to the systemic increase in response to resistance exercise,
training, which involves muscle contractions against external however, has yet to be assessed. Muscle IGF-1 mRNA and protein
loads of a wide spectrum of ranges over several weeks or a levels increase in response to both long-term resistance training
lifetime, is an effective strategy to elicit substantial gains in, or in humans and in synergist ablation-induced muscle overload
to preserve skeletal muscle mass and strength in combination in rats (Adams and Haddad, 1996; Hanssen et al., 2013). Thus,
with adequate nutrition (Morton et al., 2018; Schoenfeld even if the systemic fluctuations of IGF-1 induced by resistance
et al., 2021). Although the classically quantified adaptations exercise are negligible for the training outcome, muscle-derived
to resistance training such as increased muscle volume, larger IGF-1 may still have local and compared to non-muscle IGF-1,
cross-sectional areas of pre-existing myofibers (hypertrophy) perhaps diverging effects. For example, muscle IGF-1 may
and increased muscle force appear trivial, the spectrum of regulate muscle fiber hypertrophy or repair in a paracrine
structural and functional adaptations, especially those that manner by stimulating SC proliferation and incorporation into
underlie improvements in muscle force, is much more diverse. muscle fibers. Accordingly, increased DNA content was observed
Neuronal and neuromuscular changes (e.g., motor output), in muscles injected with IGF-1 (Adams and Mccue, 1998)
connective tissue alterations and improved vascularization—to and IGF-1 co-localization with Pax7, a marker for activated
Frontiers in Physiology | www.frontiersin.org 8 August 2021 | Volume 12 | Article 709807Leuchtmann et al. Myokines in Exercise Training Adaptation FIGURE 1 | Schematic overview of examples of the endurance (left) and resistance (right) exercise-induced muscle secretome and its auto-, para-, and endocrine actions involved in training adaptation. See text for details. Abbreviations: AMPK, AMP-dependent protein kinase; Apelin, APJ Endogenous Ligand; BAIBA, β-aminoisobutyric acid; BDNF, Brain-derived neurotrophic factor; BNP, B-type or brain natriuretic peptide; EC, endothelial cell; ECM, Extracellular matrix; GDF3, macrophage-derived growth differentiation factor 3; GDNF, glial cell line-derived neurotrophic factor, IGF-1, Insulin-like growth factor-1; ILC2, innate lymphoid cells type 2; IL-6, Interleukin-6; IL-8, Interleukin-8; IL-13, Interleukin-13; LIF, Leukemia inhibitory factor; MOTS-c, mitochondrial ORF of the 12S ribosomal RNA type-c; MMP2, Matrix metalloproteinase-2; mTORC1, mammalian target of rapamycin complex 1; NMJ, Neuromuscular junction; PGC-1α, peroxisome proliferator-activated receptor γ coactivator 1α; SPARC, Secreted protein acidic and rich in cysteine; SPP-1, secreted phosphoprotein 1; TF, transcription factor; VEGF, Vascular endothelial growth factor; YAP1, Yes-associated protein 1. SCs, was enhanced upon an acute bout of resistance exercise complexity in the regulation of IGF-1 signaling in response to (Grubb et al., 2014). However, whether SC activating IGF-1 is exercise is added by the array of IGF-1 binding proteins that can myofiber-derived in this context is difficult to estimate, as other either inhibit or promote IGF-1 bioavailability and physiological cell types are also likely to be involved. Besides SCs themselves as activity (Allard and Duan, 2018). a potential source, IGF-1 derived from monocytes/macrophages Finally, as IGF-1 is positively associated with bone is known to be crucial during ECM remodeling and regeneration mineralization (Breen et al., 2011), IGF-1 could also be from muscle injury (Tonkin et al., 2015). Another layer of involved in muscle-bone crosstalk by acting on IGF-1 receptors Frontiers in Physiology | www.frontiersin.org 9 August 2021 | Volume 12 | Article 709807
Leuchtmann et al. Myokines in Exercise Training Adaptation
in the periosteum (Rittweger, 2008; Hamrick et al., 2010). Upon secretion, myostatin binds to the activin receptor
However, even though skeletal muscle-specific overexpression of type IIB (ActRIIB) to build a complex with activin receptor-
IGF-1 results in bigger bones (Banu et al., 2003), and a potential like kinase 4 or 5 (ALK4/5) on muscle fiber membranes and
endocrine action of exercise-induced muscular IGF-1 release is SCs, which in turn activates small worm phenotype/mothers
at least conceivable, a direct link between resistance training and against decapentaplegic (SMAD) transcription factors that both
bone-specific adaptations has yet to be investigated. suppress PI3K/Akt-mediated mTORC1 signaling and induce
FoxO-dependent protein degradation pathways (Han et al.,
2013). However, how mTORC1 is regulated by myostatin
VEGF
has yet to be determined. The muscle mass increase in
Following both acute resistance exercise and hypertrophy-
myostatin KO animals results from both hyperplasia and
inducing resistance training, VEGF mRNA and protein levels
hypertrophy of muscle fibers. Hyperplasia is primarily driven
increase in skeletal muscle, indicating that VEGF plays a
during development in utero, while hypertrophy is thought to
role in the growing muscle (Richardson et al., 2000; Gavin
be the predominant process responsible for myostatin-related
et al., 2007; Kon et al., 2014). In cultured muscle cells,
postnatal muscle growth, at least in rodents and in humans.
VEGF treatment induced myotube hypertrophy, whereas VEGF
Initial studies on how myostatin negatively regulates muscle
inhibitors reduced myotube size (Bryan et al., 2008). Moreover,
mass pointed towards SC regulation, as myostatin KO mice
in mice, muscle-specific VEGF gene ablation blunts both muscle
have an increased rate of SC proliferation and incorporation
hypertrophy and strength gains in a functional overload context
into pre-existing myofibers (Mccroskery et al., 2003; Wagner
(Huey et al., 2016). Interestingly, the VEGF transcriptional
et al., 2005). However, more recent studies found that SC
response after an acute bout of resistance exercise seems to
activation upon myostatin inhibition does not precede myofiber
be attenuated in resistance-trained compared to untrained
hypertrophy and thus concluded that myostatin regulates
muscles (Richardson et al., 2000), which may indicate that
hypertrophy also in a SC-independent manner (Lee et al., 2012;
capillarization is substantially stimulated at an early stage
Wang and Mcpherron, 2012). Of note, the larger muscle mass
of training to allow subsequent muscle fiber growth, while
in myostatin KOs does not translate into increased strength
the signal flattens out when the hypertrophy rate is slowing
(Amthor et al., 2007), which might in part explain why
down. However, VEGF induction may also depend on the
pharmacological inhibition of myostatin signaling largely failed
exercise protocol as others found non-significant or a similar
to mitigate functional impairments in muscle wasting diseases
increase in acute induction of VEGF pre- and post-resistance
and sarcopenia (Rooks et al., 2020).
training (Coffey et al., 2006; Trenerry et al., 2011). It is
Antagonistic to growth hormones, myostatin contributes
currently not well understood how skeletal muscle synthesizes
to the determination of a set point of pre-programmed, yet
and secretes VEGF in response to hypertrophic stimuli, but
adaptable level of muscle mass. Inconclusive results have been
interestingly, in vitro studies found enhanced VEGF synthesis
reported regarding the regulation of myostatin expression
and secretion by C2C12 myotubes upon IGF-1 treatment and Akt
following chronic resistance exercise and the increase in muscle
activation (Takahashi et al., 2002; Florin et al., 2020). Similarly,
mass, and at least in some studies, myostatin expression
injection of constitutively active Akt into the M. gastrocnemius
even increases after acute exercise bouts (Domin et al.,
of mice enhanced both circulating and local VEGF levels
2021). It is therefore unclear whether a potential transient or
(Takahashi et al., 2002). Future studies will have to test
chronic suppression of post-exercise and post-training levels,
whether IGF-1/Akt-mediated VEGF induction plays a role in
respectively, is part of the training effect. Signals affecting the
the physiological adaptation to resistance training or whether
myostatin pathway could also be involved in resistance training
the stimuli and pathways leading to VEGF secretion and/or
adaptation. For example, resistance exercise-induced cleavage
muscle fiber capillarization in general overlap with those of
of intracellular notch domains overrides the inhibitory effect
endurance exercise.
of myostatin by inhibiting SMAD proteins (Mackenzie et al.,
2013). Finally, besides its role in keeping muscle mass in check,
Myostatin myostatin also suppresses bone development, as evidenced by
Myostatin is a highly conserved member of the TGF-β protein increased bone formation in myostatin-deficient mice (Hamrick,
family and is mainly expressed in skeletal muscle (Thomas et al., 2003; Dankbar et al., 2015). In the bone, the analogous
2000). The role of myostatin as a negative regulator of muscle ActRIIB/SMAD pathway is involved in osteoclastogenesis and
growth was first discovered in 1997 by Mcpherron et al. (1997), binding of myostatin induces osteoclast formation and thus
who observed an approximately twofold increase in muscle bone erosion (Bialek et al., 2014; Dankbar et al., 2015).
mass in myostatin KO mice. Likewise, antagonism of muscle Resistance training is a strong stimulus to boost bone mineral
myostatin signaling in mice substantially increases muscle mass, density and counteract osteoporosis, potentially at least in part
further highlighting the growth-limiting effects of myostatin mediated by reduced myostatin signaling (Zamoscinska et al.,
(Lee and Mcpherron, 2001; Amthor et al., 2004). Intriguingly, 2020), even though an ever expanding complex system of
besides its occurrence in cattle, sheep and dogs, a loss-of-function multidirectional signaling between muscle (myokines), bones
mutation in the myostatin gene may also occur spontaneously in (osteokines), liver (hepatokines) and adipose tissue (adipokines)
humans, as it happened in a young boy displaying a pronounced certainly contributes to this and other organ crosstalk in exercise
hypertrophic phenotype (Schuelke et al., 2004). (Gonzalez-Gil and Elizondo-Montemayor, 2020).
Frontiers in Physiology | www.frontiersin.org 10 August 2021 | Volume 12 | Article 709807Leuchtmann et al. Myokines in Exercise Training Adaptation
Follistatin follows a biphasic course in which initial catabolic processes,
Follistatin is a secreted glycoprotein expressed in various guided by a transient glycoprotein matrix, are followed by
cell types, including skeletal muscle (Görgens et al., 2013). anabolic processes to reinforce the intramuscular connective
Endogenous extracellular follistatin binds myostatin, thereby tissue structure (Csapo et al., 2020). Evidence further indicates
preventing myostatin-ActRIIB/ALK4/5 receptor interaction, and that long-term resistance training significantly elevates muscle
consequently inhibiting the anti-anabolic effects of myostatin MMP-2 levels and thus, a potential link between MMP-2
(Lee and Mcpherron, 2001). While myostatin loss (Mcpherron and resistance training-induced ECM remodeling of skeletal
et al., 1997) and muscle-specific follistatin overexpression (Lee muscle has been suggested (Deus et al., 2012; Shiguemoto
and Mcpherron, 2001) induce significant muscle hypertrophy on et al., 2012; Souza et al., 2014; De Sousa Neto et al., 2018;
their own, their phenotypes appear to be additive. Myostatin loss Guzzoni et al., 2018). Significantly reduced ECM remodeling
alone increases muscle mass by about twofold, but intriguingly, concomitant with impaired muscle hypertrophy and force
combined myostatin KO and follistatin overexpression results in production in MMP-2 KO mice in response to synergist ablation-
quadrupling of muscle mass (Lee, 2007), indicating that follistatin induced functional overload further supports the importance
also acts independent of myostatin. In fact, follistatin-induced of MMP-2 action during resistance training adaptation (Zhang
inhibition of SMAD3 activity, which leads to induction of protein et al., 2014). Interestingly, Akt/mTORC1 signaling is unaffected
synthesis in skeletal muscles via Akt/mTORC1/S6K signaling, in MMP2-KO mice, indicating that disruption of the ECM
occurs also through inhibition of activin, another follistatin- and remodeling alone is sufficient to reduce functional overload-
ActRIIB-ligand implicated in both muscle mass and strength loss induced muscle hypertrophy. In addition, skeletal muscle-
in various conditions (Gilson et al., 2009; Winbanks et al., 2012). secreted MMP-2 is involved in exercise-induced skeletal muscle
Although there are not many human training studies available, angiogenesis and potentially muscle-bone crosstalk, although it
both serum follistatin as well as skeletal muscle follistatin mRNA is not entirely clear whether these effects are indeed mediated
levels seem to be upregulated upon long-term resistance training by muscle-derived MMP-2 (Haas et al., 2000; Hamrick, 2012;
(Laurentino et al., 2012; Negaresh et al., 2019). The observation Chen et al., 2021).
that follistatin is not only elevated in muscle, but also in
the circulation, leads to the question whether muscle-derived
follistatin—besides controlling skeletal muscle adaptation in a
Secreted Protein Acidic and Rich in
local manner—also operates systemically. However, the bulk of Cysteine
circulating follistatin is not muscle- but liver-derived, indicating Secreted protein acidic and rich in cysteine (SPARC) is a
that endocrine follistatin is rather a hepatokine (Hansen et al., secretory matricellular protein involved in mediating cell-
2016). In this regard, a potential function of circulating follistatin ECM interactions (Brekken and Sage, 2000). SPARC expression
in glycemic control by modulating insulin action in skeletal has been described in many tissues undergoing repair or
muscle has recently been proposed (Han et al., 2019). remodeling and accordingly, both SPARC mRNA and protein
expression were elevated in human muscle biopsies after long-
term resistance training (Brekken and Sage, 2000; Norheim et al.,
Decorin 2011). Interestingly, despite being present in both myofibers
Decorin is an extracellular matrix proteoglycan involved in and capillary ECs, training-induced SPARC proteins strongly
the skeletal muscle hypertrophic response. Although in vitro accumulate adjacent to the plasma membrane of myofibers,
evidence suggests that decorin is secreted by myotubes and might indicating increased communication between muscle cells and
be involved in protein synthesis pathways, partly by inhibiting the ECM in response to resistance training (Norheim et al.,
myostatin actions similar to follistatin, the influence of a long- 2011). Although the hypertrophy-promoting signals of SPARC
term resistance training program on muscle decorin levels has are not well understood, muscle atrophy, resulting from muscle
not been studied so far (Guiraud et al., 2012; Sun et al., 2013; specific loss of SPARC, could be due to enhanced TGF-β signaling
Kanzleiter et al., 2014). (Nakamura et al., 2013). Besides its anti-atrophic effects, studies
in lens ECs provide evidence for an additional hypertrophy-
Matrix Metalloproteinase-2 promoting mechanism of SPARC. In this in vitro system, SPARC
Matrix metalloproteinases (MMPs) are a family of enzymes acts on the transmembrane receptor integrin β1, which in
involved in ECM remodeling (Nagase and Woessner, 1999). skeletal muscle plays a critical role in myofiber differentiation,
In vitro studies indicate that around 30% of skeletal muscle skeletal muscle innervation and sensing of mechanical stimuli
MMP-2 are actively secreted into the ECM and that MMP-2 has (Schwander et al., 2004; Weaver et al., 2008; Boppart and
an important function in muscular differentiation, regeneration Mahmassani, 2019). Thus, it can be speculated that SPARC
and repair (Chen and Li, 2009; Ren et al., 2019). Anomalous induces skeletal muscle hypertrophy in part through autocrine
ECM remodeling, such as connective tissue accumulation, actions via SPARC/integrin signaling. There are also other para-
affects skeletal muscle function and is often observed in aging and/or endocrine signaling events that have been attributed to
(Wood et al., 2014), while aberrant accumulation of connective SPARC e.g., in bone homeostasis, adipose tissue turnover and
tissue can be effectively prevented with resistance training tumorigenesis (Brekken and Sage, 2000; Nie and Sage, 2009).
(Guzzoni et al., 2018). Moreover, muscle ECM remodeling However, whether muscle-derived SPARC acts on these cell types
is an integral part of the resistance training response that in a para- and/or endocrine manner has yet to be investigated.
Frontiers in Physiology | www.frontiersin.org 11 August 2021 | Volume 12 | Article 709807You can also read
